1. Field of the Invention
The present invention relates generally to a bobbin on which a length of an optical fiber is wound, and, more particularly, to such a bobbin which is compliant in order to reduce the possibility of inducing tension or microbending into the fiber.
2. Description of Related Art
An optical fiber as used in telecommunications, for example, consists of a quartz core serving as the light guide that is surrounded by a polymer ("buffer") coating. When the fiber is wound into a coil on a bobbin the resulting pack exhibits a highly polytropic behavior; axial deformation due to temperature and water absorption into the buffer is relatively large while radial deformation is relatively small due to the reinforcement effect provided by the quartz fiber core. A conventional bobbin or other structure on which the fiber is wound produces a restriction in the axial deformation and any change in buffer bulk will cause undesirable fiber strain.
A quartz fiber core has a relatively high elastic modulus (at least several orders of magnitude greater than that of the buffer). The springs formed by long hoops or loops of the core on a wound pack are much more compliant than the interleaved springs formed by the comparatively thin buffer coating. As a result, any loads imposed on the fiber pack by the winding of overlying layers or the expansion of the buffer coating are reacted by changes in the quartz core hoop tension.
An example of undesirable behavior is what occurs during temperature cycling of dispensers for an optical fiber wound pack as used in certain guided missiles, for example. When heated, the metal pack bobbin expands which forces the fiber to stretch and increase pinch loading of the fiber buffer. At the higher temperature, an acceleration of relaxation ("creep") of the buffer material is experienced. Relaxation of the buffer material, in turn, causes tension in the quartz core to drop which does not, however, return to its prior levels when the dispenser assumes the original temperature. Subsequent cooling causes the metal bobbin to contract which further lowers fiber tension and introduces the threat of middle layers of a pack being driven into compression.
It is, therefore, a desideratum to provide a technique of bobbin construction which will prevent or substantially reduce tension of the quartz core of an optical fiber being induced as a result of temperature- and humidity-induced dimensional changes in the buffer coating.